Type II group B Streptococci are a frequent cause of serious human infections. They are the major cause of maternal and adult group B streptococcal infections and are second only to type III strains as a cause of neonatal infections. Antibodies directed against the type-specific polysaccharides of group B streptococci are thought to play an important role in host defense against these organisms. However, the precise nature of the immunological determinants against which protective antibodies are directed is not understood. Therefore, the complete chemical structure of the type II polysaccharide will be determined using methylation analysis, specific chemical degradations and 13C-N.M.R. The exact nature of the importance immunological determinants will be defined using various oligosaccharide fragments as inhibitors in an ELISA employing monoclonal antibodies to the type II polysaccharide. In addition, the specificity of naturally occurring antibodies to type II group B streptococci will be examined in selected human sera from patients with known colonization status or infection. Two main antibody assays will be employed: a quantitative ELISA for measuring antibody levels, and an opsonophagocytic assay for assessing the functional role of these antibodies.